Land type changing effects that change a dual land’s land type will remove the old land types completely. Text-changing effects that just change one of the two land types will leave the other type unaffected.

10/4/2004

This card is a Mountain and a Swamp even while in the graveyard, library, or any other zone.

10/1/2008

This has the mana abilities associated with both of its basic land types.

10/1/2008

This has basic land types, but it isn’t a basic land. Things that affect basic lands don’t affect it. Things that affect basic land types do.